The present invention relates to a ledge for resiliently supporting a wire screen in the region of the twin wire zone of a twin wire former of a paper making machine.
A ledge which resiliently supports a wire drainage screen or wire, is sometimes referred to herein merely as a resilient ledge. A typical twin wire former, preferably of a paper manufacturing machine, has resilient ledges installed. In the twin wire zone, a plurality of rigidly arranged ledges are provided inside the loop of one wire, for instance, at the bottom of an upwardly draining suction box. These ledges are arranged one behind the other in the direction of travel of the wire over the ledges, and generally they are rather small distances apart. All of the ledges extend transversely of the path of the wire over the entire cross machine width of the wire. Furthermore, a plurality of resilient ledges are also provided in the loop of the other wire.
The resiliency of the resilient ledges provides the following result. For instance, upon an increase of the amount of pulp suspension that is introduced between the two rotating loop wires, the resiliently supported ledges can yield somewhat. This eliminates the danger, which could occur if only rigidly supported ledges were used, of the occurrence of a back up, at the start of the twin wire zone, in the fiber suspension fed and/or in that suspension present between the two wires. Such a back up could destroy the layers of fiber in the web that had been formed up to then on one or on both wires.
The desired action is obtained, among other ways, by the resilient ledges being thrust with a predetermined force against the respective wire. This redetermined force is produced by a resilient push device, for instance, by a pneumatic pressure cushion or by several of such pressure cushions. In this way, a given draining pressure, which can increase from ledge to ledge, is established in the fiber suspension, which is still partially liquid. It is desired that a drainage pressure once established remain as far as possible constant upon a change in the thickness of the layer of suspension between the wires. The thickness of the layer of suspension can be changed by changing the amount of suspension fed or by changing the drainage behavior of the fiber suspension. Automatic adaptation of the web forming device to such changing conditions is therefore desired.
International Application WO 89/02499 discloses a resilient ledge, which includes a head ledge that is relatively wide in the direction of travel of the wire. A plurality of grooves are provided in the head ledge in effect so that the head ledge in effect forms a group of ledges. The head ledge rests on a movable support ledge having a cross section of the shape of a downwardly open U. Via two pneumatic pressure cushions, the head ledge rests on a support which is rigidly fastened in the machine.
TAPPI Proceedings 1988 Annual Meeting, pages 75-80, disclose arrangements with ledges which are relatively narrow in the direction of travel of the wire and which also serve for resiliently supporting a wire. However, those ledges are only shown diagrammatically.
Resilient ledges are also known from Federal Republic of Germany Patent 31 53 305. These ledges, however, are arranged in a curved part of the twin wire zone. At the starting end of the curved part, both wires are deflected on a rigid ledge which is part of an end wall of a box present in the opposite wire. This rigid ledge can cause a back up, as already described above. Furthermore, in each case a head ledge is connected there via a joint with a movable support ledge and that joint is subject to wear since it is exposed to the pulp water. The movable support ledges are guided in a stationary support, which is merely schematically indicated.